The present invention relates to a process for preparing precipitated calcium carbonate. According to such a method the calcium carbonate is produced by reacting calcium oxide with carbonate ions in a medium.
The invention also relates to an apparatus for preparing precipitated calcium carbonate.
The use of calcium carbonate, particularly precipitated calcium carbonate, is becoming increasingly common within many industries, such as within the paper, the plastics and the pharmaceuticals industry. The aim is to formulate of the precipitated calcium carbonate (PCC) a finely divided, pure pigment which is as white as possible for many purposes.
The production of PCC usually begins with slaked lime or calcium hydroxide which is carbonated with flue gases or subjected to causticizing using sodium carbonate or ammonium carbonate. Very finely divided pigments are often prepared by mixing calcium chloride and ammonium carbonate or soda (sodium carbonate).
The calcium hydroxide, then, is obtained by burning a calcium carbonate-containing starting material, such as limestone, in a vertical or round kiln to form calcium oxide, which is conventionally slaked to obtain calcium hydroxide. It has generally been thought that lime burning must be performed in large units such as are represented by vertical and pipe furnaces. A large unit has been seen as representing an economical process.
What is characteristic of the prior-art solutions is that all operations are carried out in different places because one customer needs burnt lime, another one slaked lime, while a third one requires reprecipitated calcium carbonate. Such operations at different stages and different locations entail transportation and storage which incur significant costs. It is characteristic of the present invention that these operations can be carried out either at the same location or, in certain situations, at different locations.
It is the aim of the present invention to remove the drawbacks hampering the prior art and to obtain an entirely novel type of solution for the preparation of precipitated calcium carbonate.
During the past few years the focus of environmental protection within the sector of power plant technology has been on removing the sulphur dioxide generated during the combustion of fuels. In this connection, much research has been carried out on the use of calcium carbonate as a sorbent which is calcinated in the power plant furnace, and the calcium oxide thus obtained reacts further with sulphur dioxide, in part directly and in part in a so called activation reactor. It has been found during these tests that the conversion of finely ground calcium carbonate into an oxide can occur in  less than 0.5 seconds at a temperature of approximately 1000xc2x0 C.
The present invention has developed these techniques further. It has namely been found possible to turn the calcium oxide directly into calcium carbonate in one process step without first having to slake it to obtain calcium hydroxide. On the other hand it has also been found that it is both technically and economically advantageous to recombine the calcium oxide and the carbon dioxide released during combustion. In short, the process of the present invention has two important embodiments whereby the first one comprises converting calcium oxide into calcium carbonate without any separate slaking or intermediate storage. In the second embodiment the calcium carbonate containing starting material is decomposed thermally and combined to form a new precipitated calcium carbonate without having to separate the products from each other for storage and without performing separate intermediate steps.
Of the above-described solutions the latter can be carried out by means of an apparatus comprising connected units for grinding limestone and for burning the ground limestone to obtain calcium oxide, a reaction unit for the calcium oxide to obtain carbonate, and a gas recovery and circulation unit. Between the units, finely divided lime or, correspondingly, calcium oxide is transferred along canals in the form of a fluid comprising solids and carrier gas.
In more detail the process according to the invention is mainly characterized by what is stated in the characterizing parts of claim 1 and, correspondingly, claim 2.
The apparatus according to the invention is characterized by what is stated in the characterizing part of claim 25.
The invention offers considerable benefits. Thus, from the point of view of thermal economy, a better solution is achieved by means of the method than by conventional lime burning, and all of the heat is recovered, even if at a lower temperature. Even the heat which is removed along with the hot product is of advantage in a northern climate in wintertime because the product can be taken to the user without being frozen.
According to the invention, the calcium carbonate powder is disintegrated into calcium oxide and carbon dioxide at an elevated temperature of approximately 800 to 1400xc2x0 C., the mixture is cooled and the final temperature is slaked with water. Next, the mixture undergoes intensive stirring, and as the end product, a 10 to 100%, for example, advantageously an approximately 60 to 80% PCC slurry is obtained directly. The solids content of the product can be adapted according to use. A product suited for use as a filler, for instance, can be prepared as an approximately 20% slurry, whereby no dispersing agents are needed. The solids content of a slurry suited for use in coating mixes may be 60 to 80%, but it is also possible to recover the PCC as a dry powder.
It is also possible to recover the calcium oxide and the carbon dioxide separately from the burning of the calcium carbonate powder. The solution is applicable both to carbon dioxide carbonating and to carbonating using known soluble carbonates, whereby, in the latter case, the carbon dioxide can be used in some other process.
One of the key ideas behind the invention is further that gas, liquid and solids are taken forward as a so called fluid, whereby the problems relating to the high viscosities and putty-like composition normally caused by the high solids contents can be controlled. In this context, a fluid is a mixture of solids and gas, possibly containing water which is present as a mist, i.e. as fine water drops. Thus, a fluid comprises a mixture of suspension and aerosol.
The starting material for lime is calcium carbonate which is converted into calcium oxide at a high temperature, the calcium oxide is slaked and calcium carbonate is formed from the calcium oxide in the same process without separating the reaction products from each other. Thus, in the inventive process, all unnecessary separation of gas and solids and water has been eliminated. Hence, according to the invention, new surface and particle shape are created in a process lasting about 3 to 10 seconds in all from the moment the calcium carbonate stone reaches the first grinder and when the same material leaves the last reaction mixer in the form of a completed aqueous slurry.
According to the invention, calcium carbonate is formed of calcium oxide in the presence of water and carbonate ions (or a precursor thereof).
According to a preferred embodiment, the calcium oxide is carbonated with carbon dioxide gas in the presence of water to form PCC. In water, the carbon dioxide forms carbonate ions. It is particularly advantageous to use such carbon dioxide gas for carbonating which has mainly been obtained from the preceding lime burning.
According to a second advantageous embodiment the PCC is prepared by reacting the calcium oxide with an aqueous solution of an alkali metal carbonate. Hereby a separately prepared alkali metal carbonate solution is fed straight into the causticizing reaction. The concentration of the alkali metal carbonate is approximately 5 to 40% by weight, whereby an alkali metal hydroxide solution having a concentration of appr. 10 to 30% by weight is obtained as a by-product of the reaction.
The calcium carbonate-containing limestone used as starting material in the invention is first ground into the desired particle size. As grinder, a pressure roll grinder or a shock grinder, for example, may be used. In general, most of the stone is ground to a particle size of less than 100 xcexcm, advantageously at least 50% of the particles are smaller than 90 xcexcm. Preferably, a great bulk are smaller than 10 xcexcm whereby essentially all silicate minerals can be separated from the limestone.
The grinding, as well as the following treatment steps, are advantageously carried out in air-free conditions, preferably in a carbon dioxide environment possibly containing water vapour and/or mist.
The powdered lime is transferred from the grinding step to preheating along a pipe equipped with a pressure-generating device. As the pressure-generating device, a suitable grinder or a corresponding device having a grinding property may be used. During preheating, the temperature of the powder is raised to above 500xc2x0 C., preferably above 700xc2x0 C., whereafter the lime is burnt in the presence of oxygen to produce calcium oxide. In the process the lime burning is performed with pure oxygen in a gaseous atmosphere in the presence of much carbon dioxide, as was stated above. This solution enables as closed a gas circulation, and therefore heat recovery, as possible, resulting in an advantageous thermal economy. The purity of the oxygen usually exceeds 80%.
In the burning of limestone, the hydrocarbon used comprises, e.g. propane, butane, kerosene, diesel oil, alcohol, vegetable oil, natural gas or biogas.
The present implementation has been made economical and therefore possible by the new advantageous methods for producing almost pure oxygen, such as molecular sieves and the PSA (Pressure Swing Absorption) method derived therefrom for separating oxygen and nitrogen.
The calcium oxide powder obtained from burning is taken via cooling further to carbonate formation which may take place through a carbonating or causticizing reaction, as stated above. In accordance with the invention it has been found that in numerous contexts the alleged precondition that the calcium oxide must be slaked before being carbonated, no longer holds true, but that the desired finely divided PCC is even more easily obtained when the slaking and the carbonating are performed simultaneously under intensive stirring. If desired, the calcium oxide can, however, also be slaked before being taken to the formation of calcium carbonate.
According to a particularly advantageous embodiment the carbonating, or, correspondingly, the causticizing, is carried out under an intensive turbulence in a turbulence zone such that the calcium oxide is reacted with water and/or water vapour via the intermediation of random pearls of carbon dioxide or carbonate compound. Thus, in the reaction the gas, liquid and solids particles are simultaneously contacted with each other under an intensive turbulence and a great energy intensity. The gas flow absorbs the liquid and the particles, forming a turbulent three-phase mixture. The solution according to the invention may also be termed a xe2x80x9cthree-phasexe2x80x9d process, as three phases are simultaneously present.
The apparatus according to the invention comprises at least two pin mills or shock grinders arranged in cascade and having one or several rotatable vane rings which can be used to impose a great energy intensity on the material fed into the apparatus. Instead of a pin mill, the carbonate reaction unit may comprise several cascaded pearl mills or successive mixing zones formed by similar mixing/grinding apparatuses. The first mixing zone is equipped with inlets for at least lime oxide, carbon dioxide and water vapour/water as well as an outlet for the reaction product, and the second one is equipped with an inlet for the product from the previous mixing zone and an outlet for the reaction product. Gas or mixing liquid can be fed between the rotating vane rings or groups of vane rings of the pin mills. The pin mills are interconnected by means of pipes which may, if desired, be furnished with inlets for the mixing liquids. The carbonating, or correspondingly, causticizing of calcium oxide takes place very rapidly in the apparatus. The residence time of the reaction is as short as less than 1 second.
The conversion of calcium carbonate increases from step to step; depending on the dry matter content of the calcium carbonate, it is usually already close to 100 after three or four steps. By dividing the method into steps, blend components can be added to the different layers of the CaCO3 particle, which components affect, among other things, the opacity and acid resistance of the product and may, on the other hand, act as dispersing agents.
According to the invention, in carbonate formation the reactants are taken to an intensive turbulence having an energy intensity of  greater than 1000 kW/m3.
The volume fraction of the calcium oxide/calcium hydroxide/calcium carbonate solution/slurry of the gas volume of the apparatus is small, typically smaller than 1%, preferably about 0.1 to 5%. By way of citing an example it may be mentioned that about 10 to 200 cm3, advantageously about 50 to 150 cm3, of calcium oxide and water or water vapour may be fed into an apparatus having a gas volume of about 40,000 cm3, and an energy of about 2,000 kW is imposed on this aerosol pre cubic metre. Due to the great energy intensity the carbonating or the causticizing may be carried out at a high solids content (10 to 100% by weight).
Any apparatus can be used for providing the turbulence, i.e. as the turbulence zone, which can be used to generate a high energy intensity in a gas volume. The apparatus used is advantageously a so called pin mill or a similar device (a shock mixer) or a pearl mill. An advantageous apparatus is described e.g. in WO Published Application No. 96/23728. The apparatus in question is mainly filled with reagent gas and only has small volumes of materials in e.g. liquid or solid phase. The condition can also be met in e.g. disk or cone refiners designed for an entirely different purpose.
The calcium carbonate obtained in the invention is of homogeneous quality. The primary diameters of the PCC particles prepared may generally be within the range from 10 to 100 nm, usually 20 to 50 nm, for minute PCC particles are produced on the surface of the lime particles during carbonating or, correspondingly, causticizing. Due to the turbulence provided by the mixer, however, these particles are separated from the surface of the calcium oxide or calcium hydroxide particles. In the mixer fluid they do not remain independent and the primary particles rapidly combine to form bigger particle aggregates or clusters of about 10 to 30, typically about 15 to 20 particles. The clusters have a size of 50 to 100 nm. The aggregates provide agglomerates or botryoidal bunches containing about 500 to 600 aggregates connected to each other. The bunches have a size of about 500 nm. They are quite strong and withstand the turbulence of the reactor. When growing bigger, looser grid agglomerates, the turbulence is reduced. The formation of these agglomerates can be carried out by adjusting the pH value such that the Z potential of the particles is as small as possible. The particles can also be used to coat other pigments, such as kaolin, chalk, talcum or titanium dioxide. The coating can be performed by feeding the pigments to be coated e.g. in the form of an aqueous slurry together with calcium oxide and carbon dioxide into the apparatus of the invention and, if necessary, by adjusting the pH to a suitable value by means of carbonic acid or some other acid (e.g. phosphoric acid).